Small water supply systems typically draw water from a reservoir of some sort and pressurise it by means of a pump which discharges the water into a plumbing circuit. Many industrial systems pump liquids other than just water by similar means. To avoid having to run the pump continuously, the pump usually discharges through a non-return valve to a plumbing circuit which incorporates a hydraulic accumulator which is a pressurised storage vessel. Also incorporated into the circuit downstream of the non return valve is a pressure sensitive switch which turns the pump on and off. The switch is designed to turn the pump on at some lower pressure and to turn it off when the pressure exceeds a higher pressure. If the flow from the plumbing circuit is sufficient, then the pressure in the circuit will not exceed the higher pressure threshold and the pump will continue to run. If a tap or other means of drawing flow from the circuit is opened, then flow will be driven through it by the liquid stored under pressure in the accumulator without the need for the pump to run. A lowered pressure is a trigger to turn the pump on.
The type of pump frequently used in such systems is a centrifugal pump with an electric motor which is either on or off. Such a system is of low cost but lacks control.
In this type of system there are problems with pressure fluctuations and associated flow fluctuations brought about by turning the pump on and off. In the case where liquid is drawn from the plumbing circuit at a high rate, the pump will not turn on until the pressure has reached the lower threshold pressure.
In the case where liquid is drawn from the circuit at a rate lower than the pump's capacity at the delivery pressure, then the pressure in the circuit will decline to the lower threshold pressure upon which the pump will be turned on and the pressure will rise to the higher threshold pressure. At this pressure, the pump will be turned off. This process may be repeated. The pressure fluctuations in the supply are annoying to the user as is the noise of the pump turning on and off. The mechanical and electrical demands of this on and off switching are deleterious to the pumping equipment.
Another problem encountered by the system described occurs when the pump loses prime. This is caused by a loss of fluid through the pump. The pump may then spin but cannot develop pressure. The only way to avoid this problem is to ensure an adequate liquid supply to the pump.
It is however desirable in some situations to totally empty the reservoir such as for cleaning purposes. After such a situation, the pump has lost prime the only way for it to regain prime is for it to be refilled with fluid. The most effective way to achieve this is to ensure that the pump has fluid available at a positive pressure at its inlet and an unrestricted outlet. By turning the pump on the air in the pump is displaced by fluid and the loss of prime situation is overcome.
Many small commercial liquid supply pumps are protected from the effects of loss of prime by a temperature sensor located on the pump motor. Continuous running caused by a failure to build pressure leads to heating of the motor. The temperature rise is detected and the motor is turned off until the temperature has dropped. This system has many limitations as the pump may still be driven in the dry condition when it is cool, thus leading to energy loss and wear of the pump seals.
Knowing the level of liquid in the reservoir is important to good pump control as loss of prime may be avoided by not drawing down the liquid level too low. It may also be used to prevent the turning on of a pump in the event that the liquid level is too low. Measurement of the liquid level has been achieved by the use of float switches, echo meters or changes in resistance or capacitance of sensors installed in the reservoir.